The invention relates to a simplification of the cabling for piezoelectric transducers of which one are more are fitted in closed systems and directly connected to electronic amplifying and switching devices.
Taking as an example an injection mould for plastics, sintered materials and light metals, the installation advantages of the invention will be shown, though it can be used for many other applications also. For the mass production of high-precision parts it is becoming increasingly necessary to fit transducers in complex production tools, in order to keep the reject rates down to a defined level.
In the so-called multicavity injection moulds up to 50 very accurate small parts can be produced in a injection cycle. Accordingly the mould is equipped with 50 piezo pressure transducers, all with different cable lengths and all connected to the signal processing in an amplifier box mounted on the actual mould. From this amplifier box, a collective cable then leads to the control and monitoring cabinet of the machine. The piezo-transducers generate highly sensitive charge signals, which are converted in the amplifier into strong voltage signals that can be led dependably to the control cabinet even in an environment with much electromagnetic interference. In general therefore, piezoelectric transducers use shielded cables having graphite interlayers to reduce electromagnetic and electrical disturbances. A disadvantage of such cables is that they can be made up into given lengths in situ only at the expense of much trouble. Another difficulty is that standard terminal blocks as generally used for normally insulated measuring lines cannot be employed with shielded cables.
The invention is intended to overcome these disadvantages when installing piezoelectric transducers and to simplify fitting them so that and connected reliably to normal terminal blocks. For this, the cables must be led completely inside the mould, so that interference fields are screened off automatically. The discovery that a piezoelectric connecting cable may be used inside a metal mould shielding is a part of the invention that was confirmed only after extensive testing. It became possible only with the availability of high-insulating terminal blocks, constituting a novelty and bringing significant simplifications to installation. Exploiting this discovery further, a transition bushing has been developed that can be fitted to the mould instead of a single terminal, enabling connection from the transducer at one end with the same unshielded cable to a normal shielded cable from outside the mould.